Geological Time
The modern science of Geology explains the solid structures of the Earth and their processes. To do this, Geologists since the beginning of the 19th Century have assumed Uniformitarianism, on which basis the Earth is approximately 4 billion years old. Prior to that time Catastrophism was frequently assumed, explaining fossils in strata as a series of creations all destroyed in turn by catastrophes. From the middle of the 20th Century the Seventh Day Adventist doctrine of Flood Geology was promoted as an alternative which harmonised with belief in a six thousand year old Earth as calculated by some Bible readers.
See The Abyss of Time — video from NASA's Earth Observatory which offers an opportunity to share a moment of realisation, observing rock formations and seeing how very ancient the Earth appears to be, as experienced by John Hutton and John Playfair one day in 1788.
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